Breast feeding might have played a criticial role in infant survival during the last ice age, and it might have led to a common genetic mutation in East Asians and Native Americans, which could have affected the shape of their teeth, says a new study.

The genetic mutation, which probably occurred 20,000 years ago, in turn led to more density of mammary ducts in the breasts, potentially providing more fat and vitamin D to infants living in the far north where the scarcity of ultraviolet radiation makes it difficult to produce vitamin D in the skin, said researchers.

If this genetic mutation is due to selection for increased mammary ductal branching, then this would be the first evidence of natural selection among the humans. "This highlights the importance of the mother-infant relationship and how essential it has been for human survival," said Leslea Hlusko, an associate professor at the University of California, Berkeley.

The gene controlling mammary duct growth also affected the shape of human incisors. The genetic mutation affected the ancestral population living in the far north during the last Ice Age, hence resulting in shovel-shaped incisors among Native Americans and northeastern Asian populations. This trait is rare in others.

The finding helps in understanding the origins of dense breasts among humans, which is a major factor behind the breast cancer.

For the study, Hlusko and her team examined the occurrence of shovel-shaped incisors in archeological populations to estimate the time and place of evolutionary selection for the trait. They found that nearly 100 percent of Native Americans prior to European colonization had shoveled incisors, and about 40 percent of East Asians today have this trait.

"When you have shared genetic effects across the body, selection for one trait will result in everything else going along for the ride," Hlusko said.

The vitamin D connection

Getting enough vitamin D is a big problem in northern latitudes because the sun is low on the horizon all year long. Sun doesn’t shine above the Arctic Circle at all for part of the year. Lack of vitamin D forced Siberians and Inuit to hunt for animal fat but babies were at a loss, thus causing the natural selection in the increased mammary duct.

Previous genetic analysis of living humans concluded that the mutation arose in northern China due to selection for more sweat glands or sebaceous glands during the last ice age but Hlusko said that it is not a satisfying explanation.

The Beringian standstill

The so-called Beringian standstill coincided with the height of the Last Glacial Maximum between 18,000 and 28,000 years ago as the climate became drier and cooler. People who had been living in Siberia moved into Beringia, where they were isolated and the species with locally adaptive traits arose.

Hlusko and her colleagues outlined many threads of evidence in their paper published in the journal Proceedings of the National Academy of Sciences.